Process for the diastereoselective synthesis of nucleoside analogues

ABSTRACT

The invention relates to a diastereoselective process for the preparation of compounds of formula (1), wherein W is S, S═O, SO 2  or O; X is S, S═O, SO 2  or O; R 1  is hydrogen or acyl, and R 2  is a purine or pyrimidine base or an analogue or derivative thereof.

This is a division of application Ser. No. 08/722,224, now U.S. Pat. No.6,051,709 filed Dec. 24, 1996 which is the national stage application ofPCT/EP95/01503 filed on Apr. 21, 1995 which is incorporated herein byreference.

The present invention relates to a diastereoselective process for thepreparation of optically active cis-nucleoside analogues andderivatives.

Nucleosides and their analogues and derivatives are an important classof therapeutic agents. For example, a number of nucleoside analogueshave shown antiviral activity against retroviruses such as humanimmunodeficiency virus (HIV), hepatitis B virus (HBV) and humanT-lymphotropic virus (HTLV) (PCT publication WO 89/04662 and EuropeanPatent publication 0349242 A2).

In particular,4-Amino-1-(2R-hydroxymethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2-one,which may be represented by the following formula:

(also known as 3TC™ or lamivudine) and its pharmaceutically acceptablederivatives, disclosed in International application PCT/GB9100706,publication no. WO91/17159, has been described as having antiviralactivity, in particular against retroviruses such as the humanimmunodeficiency viruses (HIV's), the causative agents of AIDS(WO91/17159) and hepatitis B virus (HBV) (European Patent ApplicationPublication no. 0474119).

Most nucleosides and nucleoside analogues and derivatives contain atleast two chiral centres (shown as * in formula (A)), and exist in theform of two pairs of optical isomers (i.e., two in the cis-configurationand two in the trans-configuration). However, generally only thecis-isomers exhibit useful biological activity. Therefore a generalstereoselective synthesis of cis nucleoside analogues is an importantgoal.

Different enantiomeric forms of the same cis-nucleoside analogue may,however, have very different antiviral activities. M M Mansuri et al.,“Preparation of The Geometric Isomers of DDC, DDA, D4C and D4T AsPotential Anti-HIV Agents”, Bioorg. Med. Chem. Lett., 1 (1), pp. 65-68(1991). Therefore, a general and economically attractive stereoselectivesynthesis of the enantiomers of the biologically active cis-nucleosideanalogues is an important goal.

International patent application publication no. W092/20669 discloses adiastereoselective process for producing optically active cis-nucleosideanalogues and derivatives of formula (I).

wherein

W is S, S═O, SO₂, or O;

X is S, S═O, SO₂ or O;

R₁ is hydrogen or acyl; and

R₂ is a desired purine or pyrimidine base or an analogue or derivativethereof; the process comprising the step of reacting the desired purineor pyrimidine base or analogue thereof with an intermediate of formula(IIa) or (IIb)

wherein

R₃ is a substituted carbonyl or carbonyl derivative; and

L is a leaving group;

using a Lewis acid of the formula (III)

wherein

R₅, R6 and R₇ are independently selected from the group consisting ofhydrogen; C₁₋₂₀ alkyl optionally substituted by fluoro, bromo, chloro,iodo, C₁₋₆ alkoxy or C₆₋₂₀ aryloxy; C₇₋₂₀ aralkyl optionally substitutedby halogen, C₁₋₂₀ alkyl or C₁₋₂₀ alkoxy C₆₋₂₀ aryl optionallysubstituted by fluoro, bromo, chloro, iodo, C₁₋₂₀ alkyl or C₁₋₂₀ alkoxy;trialkylsilyl; fluoro; bromo; chloro and iodo; and

R₈ is selected from the group consisting of fluoro; bromo; chloro; iodo;C₁₋₂₀ sulphonate esters, optionally substituted by fluoro, bromo, chloroor iodo; C₁₋₂₀ alkyl esters optionally substituted by fluoro, bromo,chloro or iodo, polyvalent halides; trisubstituted silyl groups of thegeneral formula (R₅) (R₆) (R₇) Si (wherein R₅, R₆ and R₇ are as definedabove); saturated or unsaturated selenenyl C₆₋₂₀ aryl; substituted orunsubstituted C₆₋₂₀ arylsulphenyl; substituted or unsubstituted C₆₋₂₀alkoxyalkyl; and trialkylsiloxy.

The process of WO92/20669 allows the stereo-controlled synthesis of aracemic cis-nucleoside analogue from an equimolar mixture of (IIa) and(IIb), and of a given enantiomer of a desired cis-nucleoside analogue inhigh optical purity if the starting material is optically pure (IIa) or(IIb). However, the W092/20669 process relies on the use of a Lewis acidof formula (III).

There are a number of disadvantages associated with the use of suchLewis acids. In particular, they are highly reactive and unstablecompounds and there are therefore hazards associated with their use.Furthermore, they are expensive and have significant toxic effects.These disadvantages are of particular importance in relation to thelarge-scale production of nucleoside analogues in factory processes.

We have now found that, by judicious selection of the leaving group L inintermediates (IIa) and (IIb), the reaction with the purine orpyrimidine base, or analogue thereof, can be successfully effectedwithout the addition of a Lewis acid catalyst, and in particular,without the addition of a Lewis acid of formula (III).

The present invention accordingly provides a stereoselective process forproducing cis-nucleoside analogues and derivatives of formula (I)

wherein

W is S, S═O, SO₂, or O;

X is S, S═O, SO₂, or O;

R₁ is hydrogen or acyl; and

R₂ is a purine or pyrimidine base or an analogue thereof; the processcomprising the step of glycosylating the purine or pyrimidine base oranalogue or derivative thereof with an intermediate of formula (IVa) or(IVb)

wherein R₃ is a substituted carbonyl or carbonyl derivative; and

G represents halo, cyano or R⁹SO₂; where R⁹ represents alkyl optionallysubstituted by one or more halo, or optionally substituted phenyl;

characterised in that the glycosylation reaction is effected without theaddition of a Lewis acid catalyst.

In a preferred embodiment, the present invention provides astereoselective process for producing cis-nucleoside analogues andderivatives of formula (I) as previously defined, which processcomprises the step of glycosylating the purine or pyrimidine base oranalogue or derivative thereof with an intermediate of formula (IVa) or(IVb) as previously defined, characterised in that the glycosylationreaction is effected without the addition of a Lewis acid of formula(III)

wherein

R₅, R₆ and R₇ are independently selected from the group consisting ofhydrogen; C₁₋₂₀ alkyl optionally substituted by fluoro, bromo, chloro,iodo, C₁₋₆ alkoxy or C₆₋₂₀ aryloxy; C₇₋₂₀ aralkyl optionally substitutedby halogen, C₁₋₂₀ alkyl or C₁₋₂₀ alkoxy; C₆₋₂₀ aryl optionallysubstituted by fluoro, bromo, chloro, iodo, C₁₋₂₀ alkyl or C₁₋₂₀ alkoxy;trialkylsilyl; fluoro; bromo; chloro and iodo; and

R₈ is selected from the group consisting of fluoro; bromo; chloro; iodo;C₁₋₂₀ sulphonate esters, optionally substituted by fluoro, bromo, chloroor iodo; C₁₋₂₀ alkyl esters optionally substituted by fluoro, bromo,chloro or iodo, polyvalent halides; trisubstituted silyl groups of thegeneral formula (R₅) (R₆) (R₇) Si (wherein R₅, R₆, and R₇ are as definedabove); saturated or unsaturated selenenyl C₆₋₂₀ aryl; substituted orunsubstituted C₆₋₂₀ arylsulphenyl; substituted or unsubstituted C₆₋₂₀alkoxyalkyl; and trialkylsiloxy.

It will be appreciated that, if the glycosylation step is carried outusing an equimolar mixture of intermediates (IVa) and (IVb), a racemicmixture of cis-nucleoside analogues will be obtained. However, it ispreferred that glycosylation is effected using an optically purecompound of formula (IVa) or (IVb), thereby producing the desiredcis-nucleoside analogue in high optical purity.

A “nucleoside” is defined as any compound which consists of a purine orpyrimidine base linked to a pentose sugar.

As used herein, a “nucleoside analogue or derivative” is a compoundcontaining a 1,3-oxathiolane, 1,3-dioxolane or 1,3-dithiolane linked toa purine or pyrimidine base or an analogue thereof which may be modifiedin any of the following or combinations of the following ways: basemodifications, such as addition of a substituent (e.g.,5-fluorocytosine) or replacement of one group by an isosteric group(e.g., 7-deazaadenine); sugar modifications, such as substitution ofhydroxyl groups by any substituent or alteration of the site ofattachment of the sugar to the base (e.g., pyrimidine bases usuallyattached to the sugar at the N-1 site may be, for example, attached atthe N-3 or C-6 site and purines usually attached at the N-9 site may be,for example, attached at N-7).

A purine or pyrimidine base means a purine or pyrimidine base found innaturally occurring nucleosides. An analogue thereof is a base whichmimics such naturally occurring bases in that its structure (the kindsof atoms and their arrangement) is similar to the naturally occurringbases but may either possess additional or lack certain of thefunctional properties of the naturally occurring bases. Such analoguesinclude those derived by replacement of a CH moiety by a nitrogen atom,(e.g., 5-azapyrimidines such as 5-azacytosine) or conversely (e.g.,7-deazapurines, such as 7-deazaadenine or 7-deazaguanine) or both (e.g.,7-deaza, 8-azapurines). By derivatives of such bases or analogues aremeant those bases wherein ring substituents are either incorporated,removed, or modified by conventional substituents known in the art,e.g., halogen, hydroxyl, amino, C₁₋₆ alkyl. Such purine or pyrimidinebases, analogues and derivatives are well known to those skilled in theart.

As used herein, halo means bromo, chloro, fluoro or iodo.

As used herein, unless otherwise stated, alkyl means straight, branchedor cyclic saturated hydrocarbon groups, or mixtures thereof.

Optionally substituted phenyl means unsubstituted phenyl or phenylsubstituted by one or more C₁₋₆alkyl, nitro, amino, halo or cyanogroups.

Preferably R₂ is a pyrimidine base. More preferably R₂ is cytosine or5-fluorocytosine.

R₃ is a carbonyl linked to hydrogen, hydroxyl, trialkylsilyl,trialkylsiloxy, C₁₋₃₀ alkyl, C₇₋₃₀ aralkyl, C₁₋₃₀ alkoxy, C₁₋₃₀alkylamine (secondary or tertiary), C₁₋₃₀ alkylthio; C₆₋₂₀ aryl; C₂₋₂₀alkenyl; C₂₋₂₀ alkynyl; or R³ is 1,2-dicarbonyl, such as

optionally substituted with C₁₋₆ alkyl or C₆₋₂₀ aryl;

or R³ is an anhydride, such as

optionally substituted with C₁₋₆ alkyl or C₆₋₂₀ aryl;

or R³ is an azomethine linked at nitrogen to hydrogen, C₆₋₂₀ alkyl orC₁₋₁₀ alkoxy or C₁₋₂₀ dialkylamino and at carbon to hydrogen, C₁₋₂₀alkyl, or C₁₋₂₀ alkoxy;

or R³ is a thiocarbonyl (C═S) substituted with hydroxyl, C₁₋₂₀ alkoxy,or C₁₋₂₀ thiol.

Preferably R₃ represents a group —C(═O)OR₄ where R₄ represents anoptionally substituted alkyl group. Preferably R₄ represents a chiralauxiliary.

The term “chiral auxiliary” describes an asymmetric molecule that isused to effect the chemical resolution of a racemic mixture. Such chiralauxiliaries may possess one chiral centre such as α-methylbenzylamine orseveral chiral centres such as menthol. The purpose of the chiralauxiliary, once built into the starting material, is to allow simpleseparation of the resulting diastereomeric mixture. See, for example, JJacques et al., Enantiomers, Racemates and Resolutions, pp. 251-369,John Wiley & Sons, New York (1981).

Preferably the chiral auxiliary R₄ will be selected from (d)-menthyl,(I)-menthyl, (d)8-phenylmenthyl, (I)-8-phenylmenthyl, (+)-norephedrineand (−)-norephedrine. More preferably R⁴ is (I)-menthyl, or (d)-menthyl,most preferably (I)-menthyl.

Preferably W is O.

Preferably X is S.

Preferably G represents halo such as Cl, Br or I, more preferably Cl,

The intermediates of formulae (IVa) and (IVb) may be isolated or theymay conveniently be generated in situ.

Suitably the intermediates of formulae (IVa) and (IVb) are generatedfrom the corresponding trans alcohols of formulae (Va) and (Vb):

wherein R₃, W and X are as previously defined, or from the epimeric cisalcohols of formulae (Vc) and (Vd):

by reaction with a reagent, suitable to introduce the group G.

Suitable reagents for introducing the group G will be readily apparentto those skilled in the art and include halogenating agents such as, forexample oxalyl bromide. Preferred halogenating agents are Vilsmeier-typereagents, which may conveniently be generated in situ by reaction of anN,N-disubstituted amide, such as dimethylformamide (DMF), and ahalogenating agent such as an oxalyl halide, e.g. oxalyl chloride, athionyl halide, e.g. thionyl chloride, a phosphorus halide, e.g.phosphorus trichloride or phosphorus oxychloride, an alkyl or phenylsulphonyl halide or anhydride. The halogenation reaction is suitablyeffected under conventional conditions.

The intermediate of formula (IVa) or (IVb) is reacted with a silylatedpurine or pyrimidine base, conveniently in a suitable organic solventsuch as a hydrocarbon, for example, toluene, a halogenated hydrocarbonsuch as dichloromethane, a nitrite, such as acetonitrile, an amide suchas dimethylformamide, an ester, such as ethyl acetate, an ether such astetrahydrofuran, or a ketone such as acetone, or a mixture thereof,preferably at elevated temperature, such as the reflux temperature ofthe chosen solvent.

Silylated purine and pyrimidine bases may be prepared as described inWO92/20669, the teaching of which is incorporated herein by reference,for example by reacting the purine or pyrimidine base with a silylatingagent such as t-butyldimethylsilyl triflate,1,1,1,3,3,3-hexamethyldisilazane, trimethylsilyl triflate ortrimethylsilyl chloride, with acid or base catalyst, as appropriate.Suitable methods are described in detail in the accompanying examples.

The cis-nucleoside analogue obtained from the reaction of the compoundof formula (I) with the purine or pyrimidine base or analogue thereofmay then be reduced to give a specific stereoisomer of formula (I).Appropriate reducing agents will be readily apparent to those skilled inthe art and include, for example, hydride reducing agents such aslithium aluminium hydride, lithium borohydride or sodium borohydride. Wehave found that stereointegrity is maintained using sodium borohydridein the presence of a phosphate or borate buffer, for example dipotassiumhydrogen phosphate, as the reducing agent.

According to the process of the invention, as well as the processdescribed in W092/20669, the final compound is typically obtained as asolution in a polar solvent, such as an aqueous solvent. This presents apractical problem in that compounds of formula (I) have a highsolubility in polar media, making their efficient isolation from suchmedia difficult. We have now found that compounds of formula (I) may beefficiently isolated from solution in polar solvents by formation of asalt having poor aqueous solubility. If desired, the water-insolublesalt may subsequently be converted to the free base, or to a differentsalt thereof by conventional methods. We have further found that thesalicylate salt is particularly suitable for this purpose.

The present invention thus provides a process as previously describedfurther comprising the step of isolating the compound of formula (I) asa water-insoluble salt, especially a salicylate salt.

Salicylate salts of compounds of formula (I) are within the scope of thepharmaceutically acceptable derivatives described and claimed inEuropean Patent Application publication no. 0382526 and publication no.W09/117159, but are not specifically disclosed therein. Such salts aretherefore novel and form a further aspect of the present invention.

In a further or alternative aspect, the present invention providessalicylate salts of compounds of formula (I), or hydrates thereof.

In particular, we have found that formation of the salicylate salt of4-amino-1-(2R-hydroxymethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2-one(lamivudine, 3TC™) affords considerable advantages for the isolation ofthat compound from polar solvents.

In a preferred embodiment the invention therefore provides4-amino-1-(2R-hydroxymethyl)-[1,3]oxathiolan-5S-yi)-1H-pyrimidin-2-onesalicylate, or hydrates thereof.

The salicylate salt of lamivudine is a pharmaceutically acceptable saltand as such it and its hydrates may be used as antiviral agents asdescribed in WO91/17159, which is incorporated herein by reference.

The salicylate salt of lamivudine or its hydrates may be formulated as apharmaceutical composition as described in WO9/117159.

The salicylate salts of compounds of formula (I) may be prepared bytreating a solution containing a compound of formula (I) with salicylicacid. Suitable solvents include for example, water and polar organicsolvents such as ethers, for example tetrahydrofuran or dioxan andalcohols, for example methanol and ethanol, or mixtures of solvents, inparticular mixtures containing an organic solvent and water.

The salicylate salts are conveniently converted, if desired, to thecorresponding free bases by treatment with a base, suitably a tertiaryamine such as, for example triethylamine.

Other suitable water-insoluble salts and methods for their preparationand conversion to free bases will be readily appreciated by thoseskilled in the art.

Intermediate alcohols (Va) and (Vb) and the epimeric cis alcohols (Vc)and (Vd) may be prepared by the methods described in WO92/20669, forexample, by reduction of the corresponding carbonyl compounds or bycondensation of an aldehyde of formula R₃—CHO, or a derivative thereof,with hydroxyacetaldehyde or mercaptoacetaldehyde, or suitablederivatives thereof. Further details of the preparation of such alcoholsmay be found in the accompanying examples.

Compounds of formulae (Va) and (Vb) are key intermediates for thepreparation of enantiomerically pure cis-nucleoside analogues orderivatives, according to the process of the invention. The absolutestereochemistry of the groups R₃, W and X in (Va) or (Vb) is preservedin the resulting cis-nucleoside analogue or derivative of formula (I).

Reactions for the preparation of alcohols of formulae (Va) and (Vb) andtheir cis epimers (Vc) and (Vd) typically result in the formation ofmixtures of isomers. When compounds of formulae (Va) or (Vb) areisolated by crystallisation from mixtures containing their enantiomersand/or their cis stereoisomers, the yield may be limited by theproportion of the desired isomer (Va) or (Vb) present in solution.

We have now found that crystallisation of the trans isomers (Va) and(Vb) is favoured over the crystallisation of the corresponding cisisomers (Vc) and (Vd). Where R₃ is an achiral moiety, a 1:1 mixture ofthe trans isomers (Va) and (Vb) may be crystallised from mixtures of thecis and trans isomers (Va), (Vb), (Vc) and (Vd).

Accordingly, the present invention provides, in a further or alternativeaspect, a method for enhancing the yield of the trans isomers (Va) and(Vb) from a mixture of the trans and cis isomers, which method comprisestreatment of the mixture of trans and cis isomers, at least partially insolution, with an agent capable of effecting interconversion of theisomers without complete suppression of the crystallisation of the transisomers.

We have further discovered that, where R₃ is a chiral moiety, a singletrans enantiomer of formula (Va) or (Vb) may be selectively crystallisedfrom a mixture of stereoisomers.

Thus, for example, compounds of formula (Va) wherein R₃ represents—C(═O)R₄, where R₄ is I-menthyl, can be selectively crystallised from amixture of stereoisomers, in particular a mixture containing alcohols(Va), (Vb) and the epimeric cis alcohols (Vc) and (Vd).

Similarly, compounds of formula (Vb) wherein R₃ represents —C(═O)R₄,where R₄ is d-menthyl, can be selectively crystallised from a mixture ofstereoisomers, in particular a mixture containing alcohols (Va), (Vb)and the epimeric cis alcohols (Vc) and (Vd).

Therefore, in a preferred aspect, the present invention provides amethod for enhancing the yield of a single enantiomer of formula (Va) or(Vb) from a mixture of isomers, which method comprises treatment of themixture of isomers, at least partially in solution, with an agentcapable of effecting interconversion of the isomers without completesuppression of the crystallisation of the desired single enantiomer (Va)or (Vb).

Agents capable of effecting interconversion of the isomers withoutcomplete suppression of the crystallisation of the trans isomersinclude, for example, alcohols, such as, for example, methanol, ethanol,n-propanol, i-propanol, n-butanol, i-butanol, t-butanol, and organicbases, in particular tertiary amines, for example, pyridine andtriethylamine and Hunig's base. A preferred agent is triethylamine.

The interconversion of isomers may be effected in any suitable solventor mixture of solvents which does not otherwise react with the alcoholsof formulae (Va) or (Vb) or their cis isomers, under conditions ofconcentration and temperature which permit crystallisation of thedesired isomer or isomers and which do not cause significant degradationof the desired isomer or isomers. Suitable solvents may include forexample, aliphatic or aromatic hydrocarbons, ethers, esters andchlorinated hydrocarbons. The interconversion will preferably beeffected at a temperature of about −20° to 120° C., more preferably inthe range of about −10° to 80° C., such as about 0° to 50° C.

It will be appreciated by those skilled in the art that selection ofsolvent, temperature, interconversion agent and, particularly, thequantity of the interconversion agent is best conducted as an integratedexercise dependent on the nature of the groups R₃, X and W present inthe isomers. However, when an organic base is used as theinterconversion agent, the preferred quantity is generally less than twomoleequivalents based on the total of all isomers of (Va) and (Vb)present.

Further guidance as to preferred reaction conditions may be gained fromthe accompanying examples.

The interconversion of isomers may be conducted separately from thepreparation of the isomeric mixture; however, it is convenientlyconducted concomitantly with that preparation.

The interconversion procedure may also be used to increase the isomericpurity of isolated (Va) or (Vb).

By means of the interconversion process, the isolated yield of thedesired isomer (Va) or (Vb) may be enhanced to greater than 50% oftheory (based on formation of all stereoisomers), typically to betweenabout 60% and about 90% of theory; but it is not ruled out that yieldsapproaching 100% of theory may be obtained.

A particularly preferred embodiment of the process of the presentinvention using I-menthol as chiral auxiliary is represented in Scheme 1and is described in detail in the accompanying examples, which are to beconstrued as illustrative of the invention and not as limiting thereof.

The invention is further illustrated by the following non-limitingexamples. All temperatures are in degrees centigrade DMSO means dimethylsulphoxide.

Example 14-Amino-1-(2R-hydroxvmethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2-one

(a) (2R,5R)-5-Hydroxy-[1,3]oxathiolane-2-carboxylic acid2S-isopropyl-5R-methyl-1R-cyclohexyl ester

A mixture of I-menthyl glyoxylate hydrate (25 g) and acetic acid (2.5mL) in toluene (125 mL) was stirred and heated to reflux. Water wasremoved by azeotropic distillation via a Dean-Stark trap. The resultingsolution of I-menthyl glyoxylate was concentrated by distillation underreduced pressure collecting ca 70mL distillate, and then cooled to20-25°. The volume was adjusted to 75 mL by adding ca 15 mL toluene,dithianediol (8.25 g) was added, and the mixture heated at reflux forabout 1 h. The mixture was cooled to about 80°, and clarified. Thefiltrate was cooled to 0-5°, and a solution of triethylamine (1.5 mL) inhexane (150 mL) was added over ca 1.25 h at 0-5°. The resultingsuspension was stirred at 0-5° for about 6 h, then the product isolatedby filtration. The product was washed with a mixture of toluene andhexane (1:3, 2×50 mL), and dried in vacuo at 40-45° to constant weight,

(b) (2R,5R)-5-(4-Amino-2-oxo-2H-pyrimidin-1-yl)-[1,3]oxathiolane-2carboxylic acid, 2S-isopropyl-5R-methyl-1R-cyclohexyl ester

A solution of (2R,5S)-5chloro[1,3]oxathiolane-2carboxylic acid,2S-isopropyl-5R-methyl-1R-cyclohexyl ester was prepared as follows:

A solution of (2R,5R)-5-hydroxy-[1,3]oxathiolane-2carboxylic acid,2S-isopropyl-5R-methyl-1R-cyclohexyl ester (300 g) in dichloromethane(3000 mL) containing methanesuiphonic acid (0.7 mL) was treated withdimethylformamide (85 mL), cooled to ca 8° and thionyl chloride (80 mL)added over ca 10 min. The resultant solution was stirred at 10-15° forca 1.5 h, then concentrated by distillation under atmospheric pressure(over ca 1.5 h), collecting ca 2.1 L distillate. The solution was cooledto 20-25°.

A solution of silylcytosine was prepared as follows:

A suspension of cytosine (115.5 g), methanesulphonic acid (0.7 mL) andhexamethyldisilazane (242 mL) was heated in toluene (290 mL) at refluxuntil a clear solution was obtained (ca 1.5 h).

The solution of silylcytosine was treated with triethylamine (145 mL),the solution of (2R,5S)-5chioro-[1,3]oxathiolane-2carboxylic acid,2S-isopropyi-5R-methyl-1R-cyclohexyl ester added maintaining a gentlereflux, washing in with dichloromethane (300 mL). The resulting mixturewas heated at reflux for 4 h, and added to a mixture of triethylamine(73 mL) and water (1200 mL) held at 30-35°, over ca 1.5 h. The resultingsuspension was stirred for ca 45 min, then hexane (1200 mL) added overca 10 min at 30-35°. The suspension was stirred at ambient temperatureovernight, then filtered. The solid was washed with water (2×600 mL) andisopropyl acetate (2×600 mL), and dried in vacuo at 40-45 °to constantweight. ¹HNMR (D₆-DMSO) δ0.75 (3H,d); 0.89(d), 0.9(m), 0.91(d),1.0-1.2(m) (9H); (9H,m); 1.43, 1.50 (2H,m); 1.67 (2H,m); 1.9-2.0 (2H,m);3.14 (1H,dd); 3.55 (1H,dd); 4.69 (1H,dt); 5.70 (1H,s); 5.80 (1H,d), 6.36(1H,dd), 7.28 (brs), 7.33 (brs) (2H); 7.97 (1H,d)

(c)4-Amino-1-(2R-hydroxymethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2-onemonosalicylate

A solution of dipotassium hydrogen phosphate (137 g) in water (150 mL)was stirred at ca 20°, and(2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-1-yl)-[1,3]oxathiolane-2-arboxylicacid, 2S-isopropyl-5R-methyl-1R-cyclohexyl ester (100 g) added. IMS (750mL) was added, and the suspension stirred for 10 min. A solution ofsodium borohydride (20 g) in water (200 mL) containing sodium hydroxidesolution, 25% w/w (2 mL) was added over 70 min, keeping the temperaturein the range 15-30°. The addition funnel was rinsed with water (50 mL),and the mixture stirred at 15-30° until the reaction was judged completeby HPLC (150 min). The mixture was allowed to settle, and the loweraqueous layer discarded. The pH of the organic phase remaining wasadjusted to 4-4.5 with conc. hydrochloric acid (27 mL), whilstmaintaining the temperature in the range 20-25°. The addition funnel wasrinsed with water (20 mL), then the pH of the solution adjusted to6.8-7.2 with 2M sodium hydroxide solution (110 mL) The addition funnelwas rinsed with water (20 mL), and the reaction mixture was transferredto a distillation vessel, washed in with water (50 mL), and the solutionheated to reflux. The solution was concentrated to ca 6.45 vol underatmospheric pressure, then cooled to 20-25°.

Menthol was removed by extraction with toluene (500 mL, 2×200 mL), theaqueous phase was diluted with water (255 mL) then treated withsalicylic acid (36 g), washing in with water (40 mL). The mixture washeated to give a solution (at 71°), then cooled to 58°. The solution wasseeded with authentic lamivudine salicylate, then cooled to 5-10° overca 4 h. The suspension was stirred for 1 h at this temperature, thenfiltered. The product was washed with water (1×100 mL, 2×200 mL), anddried in vacuo at 45-50° to constant weight. ¹HNMR (D₆-DMSO) δ_(H)3.11(dd), 3.45 (dd) (2H); 3.77 (2H,m); 5.20 (1H,m); 5.82 (1H,d); 6.22(1H,m); 6.91 (2H,m); 7.48 (1H,m); 7.62 (2H,br); 7.80 (1H,dd); 7.92(1H,d).

(d) 4-Amino-1-(2R-hydroxymethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2one

4Amino-1-(2R-hydroxymethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2-onemonosalicylate (66.7 g) was stirred with IMS (470 mL), and heated to70-75° to give a solution. The solution was clarified into acrystallisation vessel, and rinsed in with a further 170 mL IMS.Triethylamine (26 mL) was added, and the solution distilled until 280 mLremained. The solution was cooled to 70° over 20 min, seeded, thenisopropyl acetate held at 60° (600 mL) added over 2.25 h, maintainingthe temperature above 55°. The mixture was cooled to room temperatureovernight, then cooled to 8-10° and stirred for 1h. The product wasisolated by filtration (transferred to the filter with 30 mL isopropylacetate), washed with isopropyl acetate (2×130) and dried in vacuo at40-45°, to constant weight. ¹HNMR (D₆-DMSO) δ3.10 (1H,dd); 3.39 (1H,dd);3.72 (2H,m); 5.15 (1H,t); 5.29 (1H,t); 5.72 (1H,d); 6.19 (1H,dd); 7.17(1H, brs); 7.22 (1H,brs); 7.80 (1H,d).

What is claimed is:
 1. A stereoselective process for producing compoundsof formula (I)

wherein W is S, S═O, SO₂, or O; X is S, S═O, SO₂, or O; R₁ is hydrogenor acyl; and R₂ is a purine or pyrimidine base or an analogue or aderivative thereof; the process comprising the step of reacting thepurine or pyrimidine base or analogue thereof with an intermediate offormula (IVa) and (IVb):

wherein R₃ is a substituted carbonyl or carbonyl derivative; and Grepresents halo, cyano or R⁹SO₂- where R⁹ represents alkyl optionallysubstituted by one or more halo, or optionally substituted phenyl;characterized in that the reaction with the purine or pyrimidine base oranalog thereof is effected without the addition of a Lewis acidcatalyst; and with the proviso that said intermediates of formula (IVa)or (IVb) are generated from the corresponding trans alcohols of formulae(Va) and (Vb):

wherein R₃,W and X are as defined herein; or said intermediates aregenerated from the epimeric cis alcohols by reaction with a reagentsuitable to introduce the group G.
 2. A method as claimed in claim 1 forthe selective crystallisation of compounds of formula (Va) wherein R₃represents —C(═O)OR₄, where R₄ is I-menthyl from a mixture ofstereoisomers containing alcohols (Va), (Vb) and the epimeric cisalcohols.
 3. A method as claimed in claim 1 for the selectivecrystallisation of compounds of formula (Vb) wherein R₃ represents—C(═O)OR₄, where R₄ is d-menthyl from a mixture of stereoisomerscontaining alcohols (Va), (Vb) and the epimeric cis alcohols.
 4. Amethod as claimed in claim 2 for the selective crystallisation of(2R,5R)-5-hydroxy-[1,3]oxathiolane-2-carboxylic acid,2S-isopropyl-5R-methyl-1R-cyclohexyl ester.
 5. A method as claimed inclaim 4 wherein the agent capable of effecting interconversion of theisomers without complete suppression of the crystallisation of thedesired single enantiomer is triethylamine.
 6. A method for enhancingthe yield of the trans isomers (Va) and (Vb)

from a mixture of the trans and cis isomers, which method comprisestreatment of the mixture of trans and cis isomers, at least partially insolution, with an agent for effecting interconversion of the isomerswithout complete suppression of the crystallisation of the trans isomerswherein the agent is selected from the group consisting of alcohols,organic bases and Hunig's base INS wherein R₃, W, and X are defined asin claim
 2. 7. A method for enhancing the yield of a single enantiomerof formula (Va) or

from a mixture of isomers, which method comprises treatment of themixture of isomers, at least partially in solution, with an agentcapable of effecting interconversion of the isomers without completesuppression of the crystallization of the desired single enantiomer (Va)or (Vb); wherein the agent is selected from the group consisting ofalcohols, organic bases and Hunig's base wherein R₃, W, and X aredefined as in claim
 2. 8. 4Amino-1-(2R-hydroxymethyl-[1,3]oxathiolan-5S-yl)-1H-pyrimidin-2-one salicylateand hydrates this salt.